the way cpp works, is that if you dont initialize a member in the member initialization list, it gets a default constructor
so here is the new constructor

ThreeDPoint(Point p, int z) :
    p(p, z)
{}

if you dont have the member initialization list, it will automatically call the default constructor for all the members, like this

ThreeDPoint(Point p, int z) :
 p()
{}

and since Point has no default constructor (constructor with no arguments) defined, it will throw an error.

You could also fix this error by adding a default constructor to Point like so

Point() = default;

in the public scope of your class